Determining zygosity in early pregnancy by ultrasound.
D003338:Corpus Luteum / Q000000981:diagnostic imaging*; D005260:Female; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011272:Pregnancy, Multiple; D012189:Retrospective Studies; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D016216:Ultrasonography, Prenatal / Q000379:methods / Q000592:standards*